In a two-node bi-directional Free Space Optical (FSO) communication system, the two FSO nodes exchange data encoded on optical carrier beams sent across an unobstructed line of sight (LOS) between the two nodes. As shown in FIG. 1, a conventional two-node bidirectional system is illustrated. As shown, a first node 2 and a second node 3 communicate by transmitting and receiving a signal 6, 7 sent between the nodes. The data can be encoded on the signals in any matter; a binary, on-off, exemplary signal is illustrated for simplicity. Each node has an optical output 4 for transmitting the desired signal 6, 7, and also an optical input 5 for receiving the transmitted signal. Once received, the internal electronics of the node can decode the signal and obtain the transmitted data.
The communication system only works if the transmit path of the first node is aligned with the receiving components of the second node. In order to optimize tracking, conventional systems have split the received beam into two paths: one for detection and one for alignment. As shown in FIG. 1, the exemplary system uses a beam splitter and separate detectors as an alignment sensor and as a detector (processing) sensor. The resulting system is complex as it requires beam splitting and multiple paths to perform each function (e.g. alignment and detecting). Errors are also introduced into the system through the misalignment and/or drift between the multiple paths.